The present invention relates to an improved apparatus and process for cooling and de-steaming calcined gypsum stucco (calcium sulfate hemihydrate). The cooling of gypsum stucco has been of interest in the gypsum industry for many decades. Stucco is physically and thermodynamically unstable at high temperatures. In order to maintain its quality, the stucco should be quickly aerated and cooled below the temperature of calcination (250° F.). This is especially true when the calcined stucco is to be stored over time. An example of a stucco cooler is U.S. Pat. No. 6,138,377 assigned to U.S. Gypsum Company, and that disclosure is incorporated here by reference.
Hot and steamy calcined stucco stored in containers will age over time. This aging process adversely affects many of the desirable properties of the stucco, including pouring consistency, hydration rate, set time, response to accelerator, and potential for strength development. If a storage silo does not empty its contents on a “first-in-first-out” basis, the quality of the stucco removed from storage can vary drastically. Thus, the cooling and de-steaming of calcined stucco before storage is essential to long-term age stability.
Apart from age stability during storage, a further problem is caused by mechanical energy from tube mill grinding during the processing of the stucco. The mechanical energy adds more heat to the stucco, preventing many plants from being able to meet temperature requirements for bagging the stucco. Thus, immediate cooling and de-steaming of hot calcined stucco allows for stucco to be properly bagged.
Previous coolers, such as the coil cooler described in U.S. Pat. No. 6,138,377 have a high initial capital cost. Specifically, the coils and heat transfer fluid system used in a coil cooler are expensive. Also, a much larger coil cooler is required to cool the same stucco throughput compared to the invention described herein. Further, when the stucco cooled with the apparatus described herein is used to make products such as wall board, the amount of water and additives required is reduced when compared to stucco cooled in other ways. Thus, the invention described herein enjoys an economic and performance advantage over the prior art coolers.